What
is Focal length?

Focal
length is the distance from the focal point or optical centre
of the lens ( where the rays of light meet after they converge
when entering the lens) to the film plane when the lens is
focused on infinity. While this is usually measured in mm
it really relates to the angle of view or the view that
the lens projects back to the film. The focal length or distance
for a standard lens on a 35mm camera would be 50mm,
while the distance for a wide angle on the same camera could
range from 35mm to 20mm. A small telephoto lens for the same
camera might range from 80mm to 150mm. Large telephoto lenses
for the same camera format might range from 200mm up to 1000mm
or even larger.

The focal length of the lens is relevant to both digital and film cameras